The present invention relates to a method for changing the distribution of the loading pressure prevailing in the press nip of a shoe press, which shoe press comprises a number of adjacent loading elements acting on the press shoe, the first end of said elements being supported on the supporting beam of the shoe press and the other end on the press shoe.
The invention also relates to an apparatus for changing the distribution of the loading pressure prevailing in the press nip of a shoe press, which shoe press comprises a number of adjacent loading elements acting on the press shoe, the first end of said elements being supported on the supporting beam of the shoe press and the other end on the press shoe.
In paper machines, the pressing normally takes place in a press nip between press rollers, the paper web being generally passed through the press nip between water-absorbing press felts which run through the press nip together with the paper web. The length and geometric shape of the press nip have a significant effect on the pressing result.
A very efficient extended press nip is achieved by using a shoe press. The shoe press comprises a slide or press shoe, which typically has a concave pressing surface. The concave pressing surface is arranged against a backing element, such as a backing roll, and an endless belt runs between the slide shoe and the backing roll. In addition, the shoe press comprises an actuating device which presses the slide shoe against the backing roll.
As is known, the actuating device of the shoe press has a row of hydraulic loading cylinders under the show. Typically, the press shoe must be set according to the surface of the backing roll and bend according to the curvature of the backing roll surface. The press shoe must also transmit the horizontal nip forces to the supporting structures of the shoe roll. The press shoe typically assumes inside the shoe roll a spatial shape that the loading cylinder under it has to effectively follow.
On the other hand, the supporting structure under the loading cylinder bends both in the longitudinal direction MD and in the transverse direction CD of the machine, so that the supporting beam also assumes a spatial state.
In the middle part of the machine the distance between the press shoe and the supporting beam is different than in the edge areas of the machine. As a result of the overall arrangement, the opposite ends of the loading cylinder continuously assume different spatial states and the middle part is stretched due to the deflections.
Patent specification FI 103591 discloses an arrangement for moving the shoe of a shoe press.
Patent specification U.S. Pat. No. 6,083,352 discloses another approach to the loading and pull-back of the shoe of a shoe press. Solutions based on adjustment of the loading cylinder and later solutions based on adjustment of tilt for this type of cylinder are different alternatives of eccentricity. In the solution in question, the cylinders can not be mounted very close to each other due to the fastening clamps, so the loading capacity per meter of machine width is not the best possible.
Specification EP 737776 discloses a solution wherein the frame of the shoe roll contains a machined space for a loading element. A piston is fixed to the bottom of the machined space. A cylinder moves on the piston. The cylinder is continuously urged by a spring against the shoe part. The pressure inside the piston and cylinder produces the actual loading pressure. The shoe part can move in relation to the pistons. The cylinder can turn in relation to the piston.
Specification U.S. Pat. No. 5,935,385 discloses a corresponding structure in which the cylinder can move into the frame of the shoe roll in the machined space.
Specification EP 74+0016 further discloses a simple approach to solving the problem in question. In this case, the frame of the shoe roll forms a cylinder block in which the pistons are movably mounted. The upper end of the pistons leans against the loading shoe of the shoe roll, and the loading shoe can move freely in relation to the cylinder. The piston is held against the bottom of the loading shoe by means of a spring.
In specification U.S. Pat. No. 6,093,283, the piston is fixedly secured either to the loading shoe or to the frame of the shoe roll, and correspondingly the cylinder can move in relation to the shoe roll frame or the loading shoe.
A problem with all the prior-art solutions is that they provide only limited possibilities of adjustment. In addition, to make an adjustment, it has been necessary to dismantle the whole shoe press structure and only then carry out the adjustment. In prior-art solutions, typically one half of the loading element is fixedly locked to the supporting structures or to the press shoe. This imposes limitations on the adjustment.
The object of the present invention is to achieve a completely new type of solution for the loading unit of a shoe press that will allow the drawbacks of prior art to be avoided. Another object of the invention is to achieve a shoe press loading unit that will make it possible e.g. to vary the distribution of compression of the shoe press in a versatile manner. A further object of the invention is to achieve an adjustment solution that can be used without dismantling the structure of the shoe press.